In typical assemblies for drilling with downhole motors, a progressing cavity-type motor is used which has a rotor operably connected to a driven hollow shaft which supports the bit at its lower end. The fluid used to operate the motor flows through the hollow shaft and through the bit nozzles and is returned in the annulus formed by the drilling string and the wellbore. A bearing section is formed between an outer housing and the hollow shaft. The bearing section can be built as a sealed bearing section or mud-lubricated bearing section. Sealed bearing sections are used in mud- and air-drilling applications. Mud-lubricated bearing sections are mainly used in mud-drilling applications. Mud-lubricated bearing sections have limited usage in air-drilling applications.
The bearing section typically includes one or more thrust bearings, one or more radial bearings, and upper and lower seals between the outer housing and the rotating hollow shaft. Typically, to compensate for any thermal effects due to the difference between surface temperature and downhole temperatures, as well as to compensate for any entrained compressible gases in the sealed fluid reservoir surrounding the bearings, one of the seals is placed on a floating piston to allow movement to compensate for such thermal and hydrostatic effects. Some designs incorporate floating seals at both upper and lower ends of the lubricant reservoir around the radial and thrust bearings. Typical of some prior art designs involving sealed bearing systems are U.S. Pat. Nos. 4,593,774; 5,069,298; 5,217,080; 5,248,204; 5,377,771; 5,385,407; and RE 30,257.
One of the serious problems in sealed bearing sections as described above is their short life. Sealed bearing section failures can be caused by a variety of reasons, but one of the principal ones is lubrication failure. One of the main reasons for lubrication failure is overheating of the lubricant, particularly in the areas adjacent the upper and lower seals. In prior designs there has been little lubricant movement in the area of the upper and lower seals, which has resulted in undue heating of the lubricant to the point where the lubricant vaporizes and is not present in the vicinity near the end seals. This situation can create metal-on-metal rubbing and the generation of small, metallic contaminants which can engage the seals and cause their failure. Upon loss of either the upper or lower seals, the bearing assembly is no longer serviceable and drilling must stop to remove the assembly from the wellbore for repairs.
While numerous configurations of sealed bearing sections have been tried in the past, none have effectively addressed the need for more efficient lubricant circulation and cooling within the confined space of the downhole bearing section. It is, thus, an objective of the present invention to work within the confines of a typical bearing section and provide a design which will induce lubricant circulation which, in turn, enhances heat transfer from the lubricant to the circulating drilling mud in the hollow shaft and return drilling mud in the annulus. Another objective of the present invention is to incorporate the need to circulate the lubricant into the design of the radial bearing or bearings in the sealed bearing section. Yet another objective is to prolong bearing life from the typical range now experienced of approximately 80 hours of useful life to 500 hours of useful life and beyond. These and other objectives will become apparent to those skilled in the art from a description of the preferred embodiment below.